Electric lamps



United States Patent 3,213,519 ELECTRIC LAMPS Dexter P. Cooper, Jr., Lexington, Mass, assignor to Polaroid Corporation, Cambridge, Mass, :1 corporation of Delaware No Drawing. Filed Feb. 5, 1962, Ser. No. 171,292 2 Claims. (Cl. 29-1555) This invention relates to incandescent lamps adapted to be operated at relatively high filament temperatures and, more particularly, to the formation of metal carbide filament for use in such lamps.

A principal object of the present invention is to provide a method of producing coiled filaments comprising a major percentage of tantalum carbide.

Another object of the invention is to provide a method wherein a coiled filament comprising a major percentage of tantalum can be handled without undesirable coil distortion or change in the coil geometry.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the product possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

There is a demand for incandescent lamps adapted to be operated at relatively high filament temperatures and possessing relatively long, useful operating life as well as such properties as high emissivity, resistance to shock and the like. To achieve such incandescent lamps, for example, photoflood lamps, projection lamps, vehicle headlamps and other related structures adapted to project carefully controlled or substantially collimated light beams, it has been heretofore proposed to employ a filament comprising a major percentage of tantalum carbide in the presence of a regenerative atmosphere comprising a carbonaceous gas. The filaments having a suitable configuration such as, for example, a coil or coiled-coil geometry may consist essentially of tantalum carbide, such as disclosed in US. Patent 2,596,469, or they may comprise a solid solution or mixture of a major percentage of tantalum carbide and a minor percentage of at least one other refractory metal carbide such as disclosed in copending U.S. applications Serial No. 5,524, now Patent No. 3,022,436, and Serial No. 5,525, now Patent No. 3,022,437, both filed January 29, 1960. In the former application, there is disclosed and claimed the use of a carbide filament comprising tantalum carbide and from about 1 to about 30 percent by weight of at least one metal carbide selected from the group consisting of the carbides of zirconium and hafnium. In the latter-mentioned application, there is disclosed and claimed the use of a carbide filament comprising tantalum carbide and between 1 and percent by weight of at least one refractory metal carbide selected from the group consisting of the carbides of titanium, thorium, vanadium, niobium, molybdenum, tungsten and uranium.

The regenerative bulb or lamp envelope atmosphere may comprise a volatile hydrocarbon and hydrogen such as disclosed in US. Patent 2,596,469, or preferably the regenerative lamp atmosphere may comprise at least one halogen, hydrogen and carbon such as disclosed in copending U.S. applications Serial No. 840,495, filed September 10, 1959, now Patent No. 3,022,438, and Serial No. 14,254, filed March 11, 1960, now Patent No. 3,022,-

"ice

439. Other suitable regenerative atmospheres comprising at least one source of carbon and at least one source of nitrogen are disclosed in copending US. applications Serial Nos. 43,054 and 43,055, both filed July 15, 1960. Inert gases such as argon, xenon or krypton may also be used in the above regenerative atmospheres.

One convenient method of forming a preferred carbide filament comprises converting a tantalum or suitable tantalum alloy filament to the carbide structure after it has been shaped into the desired configuration or geometry. This conversion may be readily achieved by passing sufficient current through the filament to yield a filament temperature above 2000" K., preferably between about 2500 K. and 3200 K., and maintaining the filament at this temperature in the presence of a carburizing atmosphere such as, for example, hydrogen and a volatile hydrocarbon, e.g., ethylene, xylene and the like until the desired carbide structure is obtained. Carburizing techniques such as described above are more fully set forth, for example, in US. Patent 2,596,469.

In many applications such as, for instance, projection lamps, there is necessarily utilized at least one filament having a geometry, e.g., a coil or coiled-coil configuration, which enhances or provides for the obtaining of the greatest filament brightness or brilliance. Thus, after the formation of such filaments, it is essential that the geometry thereof be maintained during subsequent handling so that the desired filament brightness may be obtained. Frequently, however, during the production of tantalum carbide comprising filaments with a coil or coiled-coil structure, undesirable alteration of the coil geometry results. In other words, after the shaping of a tantalum or suitable tantalum alloy wire into the desired coil goemetry, subsequent careless handling, such as in the transport or mounting thereof, may bring about a change or alteration in the specific geometry such as, for instance, an alteration in the spacings between the coils or change in the coil diameter and the like.

In the present invention, coil distortion due to handling is eliminated or substantially minimized by handling the tantalum or tantalum alloy filament while it is wound or coiled about at least one mandrel comprising an organic polymeric material. The polymeric material may be thermoplastic in nature and must be capable of being formed into a rod or tube-shaped structure suitable for use as a filament mandrel. The organic polymeric mandrel may be removed just prior to carburization of the filament by dissolving it in a suitable solvent therefor or removed by decomposing it prior to or during carburization. Suitable filament mandrels of the present invention may be formed of thermoplastic polymeric materials such as, for example, polymethylmethacrylate or other acrylics; the cellulosics including cellulose esters and ethers such as cellulose acetate, cellulose acetate butyrate, cellulose propionate, cellulose nitrate and the like; polyamide resins such as nylon; polyalkylenes such as polypropylene etc.; polystyrene; the vinyls such as the polyvinyl alco hol, polyvinyl chloride, polyvinyl chloride-acetate copolymers, polyvinylidene chloride and the like; and others.

With any of the aforesaid polymeric materials that are not totally oxygen-free, good results may be obtained by carburizing in a hydrogen-rich carburizing atmosphere.

The organic polymeric materials employed must be such as to be formed into rigid structures of suitable configuration, for example, rod-shaped, for the shaping of a tantalum or tantalum alloy filament. In practice, a filament with a coil structure may be obtained by winding or coiling a suitable length of tantalum or tantalum alloy wire around a mandrel comprising a rod of thermoplastic organic polymeric material. Subsequent handling of the shaped filament, such as transportation, mounting operations and the like are performed while the filament is held in the desired shape on the mandrel. Prior to carburization of the shaped tantalum or tantalum alloy filament the organic plastic mandrel may be removed from the filament by dissolving it in a suitable solvent therefor or the mandrel may be retained with the shaped filament and thermally decomposed prior to or during the carburization of the filament. As noted previously, the temperatures of carburization are in excess of 2000 K. and thus sufiicient to bring about the the thermal decomposition of the various thermoplastic polymeric materials which may be utilized as the filament mandrel.

In the latter removal embodiment, the mandrel must comprise an organic polymeric material which is free of deleterious components such as, for example, oxygen, so as not to undesirably affect the carburization of the filament or the desired composition thereof. Thus, when the mandrel is to be removed by thermal decomposition during the carburization step, it preferably comprises an organic polymeric material composed only of carbon and hydrogen such as, for example, polyethylene and the like. Materials containing, in addition to carbon and hydrogen, components such as nitrogen and halogen groups may also be suitably employed in this instance. It should be noted that th thermal decomposition of the abovementioned oxygen-free polymeric materials provides a carbonaceous atomsphere suitable for aiding in the carburization of the tantalum or tantalum alloy filament.

A filament with a coiled-coil structure, for example, may be obtained in any one of several ways. For instance, a suitable length of tantalum or tantalum alloy wire may be wound around a primary mandrel comprising a rod of thermoplastic polymeric material. Prior to coiling the coiled filament about a secondary mandrel comprising a rod of thermoplastic polymeric material,

the primary mandrel may be dissolved away. The secondary mandrel, when desired, may be removed as indicated above.

Another embodiment comprises winding or coiling a suitable length of tantalum or tantalum alloy wire around a primary mandrel of a thermoplastic polymeric material to form a coiled structure and, while maintaining the primary mandrel in place, coiling the coiled filament and primary mandrel, suitably softened by heat, about a secondary mandrel comprising a rod of thermoplastic polymeric material to form the desired coiled-cool configuration. The primary and secondary mandrels may be removed by dissolving each in an appropriate solvent therefor or by thermally decomposing each prior to or during the carburization of the filament. Again, it is to be emphasized that when the latter-mentioned removal embodiment is to be utilized, the mandrel or mandrels employed be free of components such as oxygen which would be detrimental to the formation of a carbide filament of long useful operating life.

Since certain changes may be made in the above product and method without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. The method of forming a coiled filament which comprises the steps of coiling a filament comprising a major percentage of tantalum around a mandrel comprising an oxygen-free polymeric material containing carbon and hydrogen, carbnrizing said filament and simultaneously therewith removing said mandrel by thermal decomposition whereby said oxygen-free polymeric material provides a carbonaceous atmosphere which aids in the carburlzation of said filament.

2. The method of claim 1 wherein said mandrel comprises polyethylene.

References Cited by the Examiner UNITED STATES PATENTS 786,257 4/05 Beebe 29-423 2,067,746 1/37 Zabel 29423 2,142,865 1/39 Zabel 29423 2,218,345 10/40 Spaeth 2925.l4 2,359,302 10/44 Curtis 29423 2,596,469 5/52 Cooper, Jr. 3l3-223 2,790,926 4/57 Morton 29-2514 2,988,804 6/61 Tibbetts 29-l55.57

WHITMORE A. WILTZ, Primary Examiner.

JOHN F. CAMPBELL, Examiner. 

1. THE METHOD OF FORMING A COILED FILAMENT WHICH COMPRISES THE STEPS OF COILING A FILAMENT COMPRISING A MAJOR PERCENTAGE OF TANTALUM AROUND A MANDREL COMPRISING AN OXYGEN-FREE POLYMERIC MATERIAL CONTAINING CARBON AND HYDROGEN, CARBURIZING SAID FILAMENT AND SIMULTANEOUSLY THEREWITH REMOVING SAID MANDREL BY THERMAL DECOMPOSITION WHEREBY SAID OXYGEN-FREE POLYMERIC MATERIAL PROVIDES A CARBONACEOUS ATMOSPHERE WHICH AIDS IN THE CARBURIZATION OF SAID FILAMENT. 